The present invention relates generally to electrolysis cells (also referred to as electrolytic cells) and relates more particularly to electrically-conductive compression pads used in stacks of proton exchange membrane (PEM) electrolysis cells.
In certain controlled environments, such as those found in airplanes, submarines and spacecrafts, it is often necessary for oxygen to be furnished in order to provide a habitable environment. An electrolysis cell, which uses electricity to convert water to hydrogen and oxygen, represents one type of device capable of producing quantities of oxygen. One common type of electrolysis cell comprises a proton exchange membrane, an anode positioned along one face of the proton exchange membrane, and a cathode positioned along the other face of the proton exchange membrane. To enhance electrolysis, a catalyst, such as platinum, is typically present both at the interface between the anode and the proton exchange membrane and at the interface between the cathode and the proton exchange membrane. The above-described combination of a proton exchange membrane, an anode, a cathode and associated catalysts is commonly referred to in the art as a membrane electrode assembly.
In use, water is delivered to the anode and an electric potential is applied across the two electrodes, thereby causing the electrolyzed water molecules to be converted into protons, electrons and oxygen atoms. The protons migrate through the proton exchange membrane and are reduced at the cathode to form molecular hydrogen. The oxygen atoms do not traverse the proton exchange membrane and, instead, form molecular oxygen at the anode.
Often, a number of electrolysis cells are assembled together in order to meet hydrogen or oxygen production requirements. One common type of assembly is a stack comprising a plurality of stacked electrolysis cells that are electrically connected in series in a bipolar configuration. In a typical stack, each cell includes, in addition to a membrane electrode assembly of the type described above, a pair of multi-layer screens, one of said screens being in contact with the outer face of the anode and the other of said screens being in contact with the outer face of the cathode. The screens are used to form the fluid cavities within a cell for the water, hydrogen and oxygen. Each cell additionally includes a pair of polysulfone cell frames, each cell frame peripherally surrounding a screen. The frames are used to peripherally contain the fluids and to conduct the fluids into and out of the screen cavities. Each cell further includes a pair of metal foil separators, one of said separators being positioned against the outer face of the anode screen and the other of said separators being positioned against the outer face of the cathode screen. The separators serve to axially contain the fluids on the active areas of the cell assembly. In addition, the separators and screens together serve to conduct electricity from the anode of one cell to the cathode of its adjacent cell. Plastic gaskets seal the outer faces of the cell frames to the metal separators, the inner faces of the cell frames being sealed to the proton exchange membrane. The cells of the stack are typically compressed between a spring-loaded rigid top end plate and a bottom base plate.
Patents and publications relating to electrolysis cell stacks include the following, all of which are incorporated herein by reference: U.S. Pat. No. 6,057,053, inventor Gibb, issued May 2, 2000; U.S. Pat. No. 5,350,496, inventors Smith et al., issued Sep. 27, 1994; U.S. Pat. No. 5,316,644, inventors Titterington et al., issued May 31, 1994; U.S. Pat. No. 5,009,968, inventors Guthrie et al., issued Apr. 23, 1991; and Coker et al., xe2x80x9cIndustrial and Government Applications of SPE Fuel Cell and Electrolyzers,xe2x80x9d presented at The Case Western Symposium on xe2x80x9cMembranes and Ionic and Electronic Conducting Polymer,xe2x80x9d May 17-19, 1982 (Cleveland, Ohio).
In order to ensure optimal conversion of water to hydrogen and oxygen by each electrolysis cell in a stack, there must be uniform current distribution across the active areas of the electrodes of each cell. Such uniform current distribution requires uniform contact pressure over the active areas of the electrodes. However, uniform contact pressure over the active areas is seldom attained solely through design since the dimensions of the various components of a cell typically vary within some specified limits due to the production methods used in their fabrication. In fact, standard electrolysis cells often show compounded component dimensional variations of about 0.007 to about 0.010 inch due to fabrication limitations, with additional dimensional variations of up to about 0.002 inch due to differential thermal expansion during electrolysis cell operation.
One approach to the aforementioned problem of maintaining uniform contact pressure over the entire active areas of the electrodes has been to provide an electrically-conductive compression pad between adjacent cells in a stack. One type of electrically-conductive compression pad that has received widespread use in the art comprises an elastic disk, said disk being provided with an array of transverse holes and transverse slots. The transverse holes are provided in the disk to allow for lateral expansion during compression of the disk. The transverse slots are provided in the disk so that a plurality of parallel metal strips may be woven from one face of the disk to the opposite face of the disk through the slots.
Other types of electrically-conductive compression pads are disclosed in the following patents, all of which are incorporated herein by reference: U.S. Pat. No. 5,466,354, inventors Leonida et al., issued Nov. 14, 1995; U.S. Pat. No. 5,366,823, inventors Leonida et al., issued Nov. 22, 1994; and U.S. Pat. No. 5,324,565, inventors Leonida et al., issued Jun. 28, 1994.
Compression pads of the type described above comprising an elastic disk having parallel metal strips woven therethroughout are generally capable of compensating for dimensional variations of a cell to maintain uniform contact over the active areas of the cell up to pressures of about 500 psi. Unfortunately, however, more and more applications require increased gas delivery pressure capabilities. Increased pressure requirements were initially addressed by enclosing the entire cell stack within a pressure vessel to limit the maximum load across the pressure pad to about 200 psi. In such a configuration, the compression pad was vented to the vessel, and the stack was operated in a balanced pressure mode, i.e., both gases were produced at approximately the same pressure. The vessel plus the pressure controls associated with this configuration, however, added significant weight and expense to the system.
Electrolysis cell stacks without a pressure vessel are simpler, lighter, and less expensive than those requiring pressure vessels. In such a configuration omitting a pressure vessel, the compression pad is totally sealed, i.e., not externally vented, and must withstand significantly higher pressure differentials, approximately equal to the sum of the highest internal pressure during operation and the compression required to maintain uniform contact. This pressure differential can reach about 2,500 psi or greater. As can readily be appreciated, such pressure differentials render compression pads of the type described above only marginally useful.
It is an object of the present invention to provide a novel electrically-conductive compression pad suitable for use in an electrolysis cell stack.
It is another object of the present invention to provide an electrically-conductive compression pad of the type described above that overcomes at least some of the shortcomings discussed above in connection with existing electrically-conductive compression pads.
It is still another object of the present invention to provide an electrically-conductive compression pad of the type described above that can be mass-produced, that is easy to manufacture and that is simple to use.
Therefore, in furtherance of the above and other objects to be described or to become apparent from the description below, there is provided herein an electrically-conductive compression pad suitable for use in an electrolysis cell stack, said electrically-conductive compression pad being constructed according to the teachings of the present invention and comprising (a) a single sheet of electrically-conductive material, said single sheet of electrically-conductive material having a top surface and a bottom surface; and (b) elastomeric material arranged on said single sheet of electrically-conductive material in such a way that, when said elastomeric material is compressed, substantially uniform pressure is exerted across each of said top surface and said bottom surface of said single sheet.
With respect to the aforementioned compression pad, the elastomeric material is preferably arranged on each of said top and bottom surfaces of said single sheet, and said single sheet is preferably bent to lie flush with said elastomeric material at one or more points on each of said top and bottom surfaces when said elastomeric material is compressed. Said single sheet of electrically-conductive material is preferably a sheet of metal, said metal preferably being, but not being limited to, a low resistivity (or high conductivity) metal selected from the group consisting of niobium, titanium, zirconium, tantalum, copper, nickel, hastelloy, and steel. Said single sheet may be circular or rectangular in shape but is not limited thereto. The elastomeric material is preferably a rubber having a shore A durometer of approximately 45 to 100. For example, where the compression pad is intended for use at pressures of up to about 800 psi. the elastomeric material may be a silicone rubber having a shore A durometer of about 55; alternatively,. where the compression pad is intended for use at pressures of up to about 2500 psi, the elastomeric material may be a polyurethane having a shore A durometer of about 95.
According to another aspect of the invention, there is provided an electrically-conductive compression pad suitable for use in an electrolysis cell stack, said electrically-conductive compression pad comprising (a) a single sheet of electrically-conductive material, said single sheet of electrically-conductive material having a top surface and a bottom surface, said single sheet of electrically-conductive material being bent up and down to include a plurality of alternating ribs and channels; and (b) elastomeric material mounted within said channels, said elastomeric material being dimensioned so that, when said elastomeric material is compressed, said elastomeric material lies flush with said ribs and exerts substantially uniform pressure across each of said top surface and said bottom surface of said single sheet.
With respect to the aforementioned compression pad, the alternating ribs and channels are preferably linear and parallel to one another. The ribs on the top surface of said single sheet form channels on the bottom surface of said single sheet and vice versa. Said single sheet is preferably a sheet of metal, said metal preferably being, but not being limited to, a low resistivity (or high conductivity) metal selected from the group consisting of niobium, titanium, zirconium, tantalum, copper, nickel, hastelloy and steel. Said single sheet may be circular or rectangular in shape but is not limited in shape thereto. The elastomeric material is preferably a rubber having a shore A durometer of approximately 45 to 100. For example, where the compression pad is intended for use at pressures of up to about 800 psi, the elastomeric material may be a silicone rubber having a shore A durometer of about 55; alternatively, where the compression pad is intended for use at pressures of up to about 2500 psi, the elastomeric material may be a polyurethane having a shore A durometer of about 95.
According to still another aspect of the present invention, there is provided an electrolysis cell stack, said electrolysis cell stack comprising (a) a first electrolysis cell; (b) a second electrolysis cell, said second electrolysis cell being arranged in series with said first electrolysis cell; and (c) an electrically-conductive compression pad of the type described above interposed between said first electrolysis cell and said second electrolysis cell.
Additional objects, features, aspects and advantages of the present invention will be set forth, in part, in the description which follows and, in part, will be obvious from the description or may be learned by practice of the invention. In the description, reference is made to the accompanying drawings which form a part thereof and in which is shown by way of illustration specific embodiments for practicing the invention. These embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.